The demand for rechargeable batteries having ever greater energy density has resulted in substantial research and development activity in rechargeable lithium batteries. The use of lithium is associated with high energy density, high battery voltage, long shelf life, but also with safety problems (ie. fires), since lithium is a highly reactive element. As a result of these safety problems, many rechargeable lithium battery electrochemistries and/or sizes are unsuitable for use by the public. In general, batteries with electrochemistries employing pure lithium metal or lithium alloy anodes are only available to the public in very small sizes (eg. coin cell size) or are primary types (eg. non-rechargeable). However, larger rechargeable batteries having such electrochemistries can serve for military or certain remote power applications where safety concerns are of somewhat lesser importance, or the personnel involved are trained to deal with the higher level of hazard.
Recently, a type of rechargeable lithium battery known as lithium-ion or `rocking chair` has become available commercially and represents a preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable battery systems (ie. NiCd, NiMH, or lead acid batteries). Additionally, the operating voltage of lithium ion batteries is often sufficiently high that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. 3.6 V (average) lithium ion batteries based on LiCoO.sub.2 /pre-graphitic carbon electrochemistry are now commercially available. Many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke and pure graphite. The aforementioned products employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
Lithium ion batteries can be sensitive to certain types of abuse, particularly overcharge abuse wherein the normal operating voltage is exceeded during recharge. During overcharge, excessive lithium is extracted from the cathode with a corresponding excessive insertion or even plating of lithium at the anode. This can make both electrodes less stable thermally. The anode becomes less stable as it gets doped or plated with reactive lithium while the cathode becomes more prone to decomposing and evolving oxygen (see J. R. Dahn et al., Solid State Ionics, 69(3-4), p265-270, 1994). Overcharging also results in heating of the battery since much of the input energy is dissipated as heat rather than stored. The decrease in thermal stability combined with battery heating can lead to dangerous thermal runaway and fire on overcharge.
Battery chargers and/or battery packs comprising assemblies of individual lithium ion batteries are generally equipped with appropriate electrical circuitry to prevent overcharge. However, in the event of failure of the circuitry, many manufacturers incorporate additional safety devices, in the individual batteries themselves, to provide a greater level of protection against overcharge abuse. For instance, as described in U.S. Pat. No. 4,943,497 and Canadian Patent Application Ser. No. 2,099,657, filed Jun. 25, 1993, published Feb. 11, 1994, respectively, the lithium battery products of Sony Corporation and Moli Energy (1990) Limited incorporate internal disconnect devices which activate when the internal pressure of the battery exceeds a predetermined value during overcharge abuse. Various gassing agents (eg. cathode compounds and/or other battery additives) may be used to generate sufficient gas above a given voltage during overcharge so as to activate the disconnect device.
Another alternative method relies on the net increase in internal solids volume to hydraulically activate a disconnect device at a specified state of overcharge (as disclosed in Canadian Patent Application Ser. No. 2,093,763, filed Apr. 8, 1993, published Oct. 9, 1994).
Other overcharge safety devices may be incorporated in the lithium batteries themselves to limit the charging current and/or voltage. Positive temperature coefficient resistors (PTCs) are incorporated by some manufacturers in part to limit the charging current during overcharge abuse. These devices rely on a combination of heating of the battery and IR heating of the PTC to trigger the PTC, which thereby increases its resistance and limits the charging current. In principle, it is also possible to consider incorporating an electrical circuit for overcharge protection in the headers of the individual batteries themselves.
These additional or backup safety devices can be effective insofar as eliminating hazards associated with the electrical abuse of overcharge. However, the overcharged battery is typically left in a higher state of charge than normal. The contents of the battery can therefore be left in a less than normal thermally stable state, thereby posing more of a hazard than normal. Such overcharged batteries can be more sensitive to subsequent mechanical abuse (eg. being crushed) or thermal abuse (eg. being heated in an oven). While many batteries can simply be discharged manually in the event that overcharge abuse has occurred, thereby placing the battery in a safe discharged state for later disposal, it is preferred that this discharge be done automatically.
Batteries with activated internal electrical disconnect devices however cannot be externally discharged to drain them of energy and lower the state of charge. Such disconnected batteries may be locked into an abnormally unsafe state of charge and pose additional risk with regards to disposal or tampering. Unfortunately, after the activation of a disconnect, such a battery will appear to have no remaining capacity (ie. be completely dead). At this point, an unwary consumer might be more tempted than usual to disassemble or otherwise mechanically abuse the battery with unfortunate consequences as a result. Thus, means for discharging such overcharged batteries automatically and internally are highly desirable.
Several means for automatically discharging batteries are known or have been proposed in the art. Aqueous battery electrochemistries may exhibit recombination reactions at the end of charge which effectively serve to continuously discharge the battery while charging continues. Additives (chemical shuttles) have also been disclosed for non-aqueous battery electrochemistries to serve a similar purpose. Recombination reactions and chemical shuttles may be viewed as automatically discharging the batteries but only such that the normal maximum operating charging voltage is not exceeded.
Means for creating internal short circuits in overcharged batteries are also known in the art. Electrochemical corrosion reactions may be relied on to rapidly corrode metallic hardware or other additives which are maintained at cathode potential (eg. cathode current collector). A corroded species from the cathode can then migrate and plate at the anode resulting in the formation of a conductive dendrite. With continued corrosion and plating, a conductive dendrite bridge can form between the cathode and anode thereby electrically shorting the battery through the dendrite bridge. Often, little actual charge needs to be consumed in corrosion reactions before a dendrite bridge forms. Thus, cathode hardware materials or other additives may be suitable for this purpose if the onset of corrosion occurs above the maximum operating voltage and if significant corrosion occurs before overcharging presents a safety hazard. Many readily available material options exist for low voltage (eg. circa 2 volt) non-aqueous batteries. For instance, in lithium anode/molybdenum disulfide cathode batteries manufactured by Moli Energy Ltd. in the 1980s, stainless steel and/or nickel hardware at cathode potential would corrode, create dendrite bridges, and short circuit the battery internally thereby limiting the state of charge and protecting the batteries during overcharge abuse. However, not so many material options are available for higher voltage (eg. circa 4 volt) non-aqueous batteries. Most commonly available hardware materials corrode at too low a potential to allow for the normal operation of the battery. On the other hand, those speciality materials which do not corrode at too low a potential may not corrode significantly enough when needed for overcharge protection. Thus, neither common nor speciality materials are readily available for higher voltage non-aqueous batteries. `
Mechanical means for creating internal short circuits in overcharged batteries have also been considered in the art. For instance, one option proposed is similar to the aforementioned electrical disconnect devices except that instead of effecting a disconnect when activated, a mechanism would instead be incorporated which effected a short circuit connection. This option however is mechanically complex and raises cost and reliability concerns.
Ideally, the means for creating internal short circuits on overcharge would be reliable and inexpensive. Optimally, mild shorts are produced, perhaps progressively or incrementally and perhaps distributed throughout the inside of the battery, such that the power and heat dissipated through the shorts is not suddenly large or localized (ie. creating spot heating). Either of these latter conditions represents a hazard in themselves.
Co-pending Canadian Patent Application Ser. No. 2,163,187, filed Nov. 17, 1995, by a common inventor, discloses the use of polymerizable monomer additives as gassing agents in lithium batteries for purposes of activating internal electrical disconnect devices on overcharge. Therein, it is disclosed that certain monomer gassing agents which form conductive polymer products might provide the additional advantage of creating an internal short and discharging the batteries following overcharge abuse. In the examples, this additional advantage is actually obtained in batteries comprising a biphenyl additive. The polymerization product of the biphenyl is conductive.
Co-pending Canadian Patent Application Ser. No. 2,156.800, filed Aug. 23, 1995 by a common inventor, discloses the use of polymerizable monomer additives for purposes of protecting a rechargeable lithium battery during overcharge. Therein, a small amount of polymerizable additive is mixed in the liquid electrolyte. During overcharge abuse, the aromatic additive polymerizes at voltages greater than the maximum operating voltage of the battery thereby increasing its internal resistance sufficiently for protection.
In the aforementioned co-pending Canadian patent applications Ser. Nos. 2,163,187 and 2,156,800, it is not directly disclosed that it would be advantageous in general to have batteries automatically discharge themselves after overcharge abuse, ie. independent of whether the battery contained an internal disconnect device. Also, it is not directly disclosed that the use of monomer additives which form conductive products when polymerized can be advantageous independent of whether the monomer also serves as a gassing agent or serves to significantly increase the internal resistance of the battery.
Some aromatic compounds which are fundamentally capable of polymerizing electrochemically and forming conductive polymers have been used in electrolyte solvent mixtures and/or as electrolyte solvent additives in certain specific rechargeable non-aqueous lithium batteries for purposes of enhancing cycle life. In Japanese Patent Application Laid-open No. 61-230276, a laboratory test cell employing an electrolyte comprising a furan (an aromatic heterocyclic) solvent additive demonstrated an improved cycling efficiency for plated lithium metal. In Japanese Patent Application Laid-open No. 61-147475, a polyacetylene anode, TiS.sub.2 cathode battery employing an electrolyte comprising a thiophene solvent additive showed better cycling characteristics than similar batteries without the additive. No mention is made in these applications about potential safety advantages resulting from the electrochemical polymerization capability of the additives. Also, it is unclear whether the actual embodiments in these applications would possess a safety advantage in practice during overcharge abuse as a result of incorporating the additives (ie. other events that occur during overcharge might prevent polymerization and/or polymerization might not result in the creation of an internal short).